Transformer oil acts as an insulator and coolant. Regular testing helps detect insulation breakdown, contamination, and early faults. Below are the essential tests for transformer oil:
—
1. Dielectric Strength (BDV – Breakdown Voltage Test)
This test measures the oil’s ability to withstand high voltage without breaking down.
Test Equipment:
BDV (Breakdown Voltage) Test Kit
Stirring Rod (if manual test setup)
Test Cup (400 ml)
Procedure:
1. Sample Collection – Take a sample in a clean, dry container.
2. Fill the Test Cup – Pour oil between two electrodes (2.5 mm or 4 mm apart).
3. Apply Voltage Gradually – Increase voltage at 2 kV/sec.
4. Record Breakdown Voltage – The voltage at which an arc forms between electrodes.
5. Repeat Test – Conduct at least 5 breakdown tests, taking the average value.
Acceptance Criteria:
New oil: ≥ 30 kV
In-Service oil: ≥ 22 kV
If BDV is low, oil filtration or replacement is needed.
—
2. Dissolved Gas Analysis (DGA)
DGA detects gases produced due to overheating, arcing, or insulation degradation.
Common Gases and Their Indications:
Hydrogen (H₂) → Partial discharge
Methane (CH₄), Ethylene (C₂H₄) → Overheating of oil
Acetylene (C₂H₂) → High-energy arcing
Carbon Monoxide (CO), Carbon Dioxide (CO₂) → Paper insulation breakdown
Procedure:
1. Collect a gas-tight oil sample in a syringe or glass container.
2. Analyze with a Gas Chromatograph (GC).
3. Compare results using Duval Triangle or Rogers Ratio Method to diagnose faults.
—
3. Moisture Content (Water in Oil – ppm Level)
Excess moisture reduces insulation properties and accelerates aging.
Test Methods:
Karl Fischer Titration (Accurate method)
Capacitive Sensor (On-site rapid test)
Acceptance Criteria:
New oil: ≤ 35 ppm
In-Service oil: ≤ 50 ppm
If moisture is high, vacuum dehydration is required.
—
4. Interfacial Tension (IFT) Test
Measures oil’s ability to resist contamination by polar compounds (aging indicator).
Procedure:
1. Mix transformer oil with distilled water.
2. Measure the force required to separate the two layers.
Acceptance Criteria:
New oil: ≥ 40 dynes/cm
Bad oil: ≤ 25 dynes/cm → Requires replacement.
—
5. Acidity Test (Neutralization Number)
Indicates oil degradation due to oxidation.
Procedure:
1. Mix transformer oil with KOH (potassium hydroxide) solution.
2. Measure neutralization number (mg of KOH needed to neutralize acid in 1g oil).
Acceptance Criteria:
New oil: ≤ 0.03 mg KOH/g
Bad oil: ≥ 0.2 mg KOH/g → Requires oil change.
—
6. Flash Point & Pour Point
Flash Point → Minimum temperature at which oil vapors ignite (≥ 140°C is safe).
Pour Point → Minimum temperature where oil solidifies (Should be low for cold regions).
—
7. Furan Analysis (Paper Insulation Degradation Test)
Measures 2-Furfural compounds in oil.
High levels indicate paper insulation aging.
Test Procedure:
1. Extract furan from oil sample.
2. Analyze using High-Performance Liquid Chromatography (HPLC).
Acceptance Criteria:
< 0.1 ppm → Good
> 1 ppm → Severe paper degradation
—
Conclusion
Regular transformer oil testing helps detect early faults and prevents failures. If test results indicate contamination or aging, oil filtration, dehydration, or replacement is necessary.
Electtechknownews 
Leave a comment